The Day Two Advantage: A Retrospective Paired Analysis of Consecutive <scp>HPC</scp>(A) Collections

The hematopoietic progenitor cell apheresis [HPC(A)] is a cornerstone of stem cell transplantation. While multiple apheresis sessions are common in clinical practice, few studies have examined the procedural variability within the same donor across consecutive collections. This study aims to compare changes in the efficiency metrics between the first and second HPC(A) procedures. A retrospective analysis was conducted on paired HPC(A) procedures performed on the same donor between January 2023 and May 2025 at the Abu Dhabi Stem Cells Center. The predefined parameters, including the collection efficiencies (CE₁, CE₂, cruCE), performance ratio (PR), and total volume processed (TVP), were compared using the Wilcoxon signed-rank tests. Effect sizes were calculated using Cohen's r, and associations between variables were assessed using the Spearman correlation. Of the 215 procedures screened, 90 met the inclusion criteria. Despite similar preapheresis CD34⁺ cell counts and flow rates, second-day procedures demonstrated significantly improved CE₁, CE₂, and PR, with lower TVP. A large effect size was observed for the TVP/total blood volume (TBV) ratio (r = 0.63), and moderate-to-large effect sizes for CE₂ (r = 0.41), PR (r = 0.47), and CE₁ (r = 0.47). CE₁ and CE₂ correlated strongly with PR across both sessions, while CE₂ correlated positively with TVP/TBV on day 1 (ρ = 0.47) and negatively on day 2 (ρ = -0.40). Second-day HPC(A) sessions yielded superior efficiency despite reduced volume, supporting the value of individualized collection planning. These "day two advantages" may inform refinements in yield prediction and highlight the need for prospective studies to optimize apheresis strategies.